Telecommunication networks must be properly maintained to ensure that adequate network performance is achieved and that end-user services are supported. Maintenance functions include "performance management" (continuous in-service performance monitoring for proactive warning of performance degradation) and "fault management" (detection and location of network trouble and failure).
Delay monitoring is important in managing performance of ATM networks and the following parameters are used for such purposes because they affect important network management functions.
Cell Transfer Delay (CTD)
Relates to throughput and response time for high speed data services, and is used for:
provisioning congestion and protocol parameters such as window sizes and time-outs; PA1 selecting low delay routes (e.g. to avoid satellite links); and PA1 deploying echo cancellers. PA1 dimensioning AAL-1 buffers for smoothing CBR (continuous bit rate) traffic; PA1 detecting excessive traffic; and PA1 predicting congestion. PA1 If "the clocks of the BSSs are synchronized in absolute time, . . . the one-way delay can be measured directly with a Performance Management OAM cell. However while the frequencies of the BSSs' clocks will be almost perfectly matched in a BISDN network, the absolute time is not expected to be synchronized. In practice, absolute time differences of several seconds are possible. PA1 Whether the clocks are synchronized or not, there is a lower bound on the delays observed at a receiving node. Delays longer than the minimum would be caused by queuing and processing delays. . . . the parameter of interest is how many delay measurements exceed the maximum allowed value, L+V.sub.max, where L is the lowest observed value (obtained through calibration). . . . PA1 When the timestamp is being used, it is encoded in the PM OAM cell at the originating end. This time stamp will be accurate to within .+-.1.0 .mu.sec. The terminating end point compares the time stamp to the time shown by its own clock. This comparison needs to be done as soon as OAM processing has begun on the received PM OAM cell, so that the delay measurement includes as little OAM cell processing time as possible. Variation of the delay experienced by the PM OAM cell will provide a good estimate of the delay variation experienced by the user-information cells. PA1 . . One can estimate the lowest value, L, by a calibration procedure in which the delays of the first C PM cells [C may be e.g. 1000] are observed, and the lowest value is recorded. Note that L may be negative, because the clocks of the two nodes are not necessarily synchronized. The amount by which the observed delay measurements exceed L provides an unbiased estimate of the delay variation." PA1 the originating mode must encode time stamps, PA1 the receiving node must calibrate the first C PM cells to calculate L, and PA1 the receiving node must count the number of PM cells with delays greater than L+V.sub.max."
Cell Delay Variation (CDV)
Used for:
To support performance and fault management functions of VPC/VCC (virtual path connection/virtual channel connection) in ATM networks, OAM cells are defined to carry operation information such as error checks, node identifiers (IDs), fault descriptions, loopback indications, timestamps, etc. OAM cells are identified in the ATM cell header as separate from user cells.
Bellcore Technical Advisory TA-NWT-001248, Issue 1, October 1992, describes on pages 5-12 and 5-13 how Performance Management OAM cells (PM OAM cells), each containing a timestamp, can be used to obtain an estimate of excessive cell transfer delay occurrences at the broadband switching system that receives the timestamp information in the forward report within the forward monitoring cell. It further states that this count can only be made and stored at the connection/segment end point that receives the forward monitoring cell, because at present there is no field in the PM OAM cell that allows backward reporting of excessive cell transfer delay occurrences. Bellcore goes on to state:
To summarize, Bellcore states that:
"To measure cell delay variation, the following actions have to be performed:
Monitoring can be performed at different locations in a network and the following are examples:
a) Near-End monitoring which provides performance of a received signal from its origination to its termination. Bit Interleaved Parity (BIP) is used for ATM by forward monitoring OAM cells. The monitoring point is at the received signal termination.
b) Far-End monitoring provides performance of a transmitted signal from its origination to its termination. For ATM, performance at the far-end termination is sent back to the monitoring point in received signal overhead, e.g. backward reporting OAM cells. The monitoring point is at the received signal termination where the overhead is read.
c) Intermediate monitoring is at intermediate locations in a transparent mode such that near- and far-end performance indicators are read but not terminated. This provides performance of the received signal from its origination to the intermediate monitoring point (e.g. by calculating BIP in forward monitoring OAM cells), and performance of a transmitted signal from its origination to its termination (e.g. by reading backward reporting OAM cells at the intermediate monitoring point).
As seen in the above description, the technique described by Bellcore only provides near-end monitoring, and for one parameter only. The present invention provides near-end and/or far-end monitoring of a number of delay parameters of a telecommunication network such as an ATM or frame relay network.
The present invention can therefore support single-ended monitoring, that is to say, it can monitor performance in both directions from one end. The present invention can further support single-ended monitoring from each node so that both nodes can obtain results of their far-end as well as near-end monitoring.